The two key factors for the generation of the diverse antigen receptor repertoire in the human adaptive immune system are the products of the recombination activating genes 1 and 2 (RAG1 and RAG2). The identification of their homologs, spRAG1L and spRAG2L respectively, in the genome of the purple sea urchin, Strongylocentrotus purpuratus was unexpected as there is no evidence thus far of an adaptive immune system in invertebrates (including echinoderms), i.e. they lack at least one of its hallmarks, a diversified antigen receptor repertoire. To study the expression pattern of both proteins and to identify when and where they are present during development, we generated and affinity purified polyclonal antisera against both, spRAG1L and spRAG2L. All our antisera are highly specific in western blot experiments, and their performance in immunoprecipitation and immunofluorescence experiments are currently under investigation. In addition, we started characterizing the biochemical properties of spRAG2L. The C-terminus of this protein is predicted to adopt a plant homeo domain-like (PHD) finger fold, a conserved zinc-binding structure that has been shown to interact with histone tails. Preliminary experiments revealed that the PHD domain of spRAG2L, similar to the respective domain of murine RAG2, is indeed able to bind specifically to histone H3 tails. In these assays, the respective domain of spRAG2L showed strong preference for peptides with a dimethyl modification of lysine 4 (H3K4diMe). This observation supports our hypothesis that the sea urchin RAG proteins indeed act on DNA in the nucleus, and that access to defined regions in the sea urchin genome with native chromatin structure might be regulated at the level of histone modifications. Our studies have major implications for the current model of how adaptive immunity evolved in jawed vertebrates, and will help to illuminate conserved features of how V(D)J recombination is tightly controlled to avoid potentially dangerous modifications of the genome.